This invention relates to the design of variable pitch fans used on engines, particularly heavy machinery.
The design of a variable pitch fan involves a number of difficult design challenges. First, there is the problem of obtaining high performance without sacrificing high efficiency in converting energy into air flow. Since the energy required to move air is a function of the square of the air velocity, the theoretically most efficient way to move the air is to develop a flat velocity profile across the fan. This keeps the velocity of the air near the center of the hub the same as at the outer edges. Because the blades are rotating, the velocity of the blade increases away from the center of rotation. Therefore in order to maintain a flat velocity profile (the axial velocity of the air across the fan diameter), the fan designer needs to increase the pitch and the section (size of the cross section) of the blade near the center where the blade is moving slowly. At the perimeter, there are much higher speeds, and so the fan has similar performance with a smaller, less pitched profile. The conventional twist and taper of variable pitch fan blades in which the fan blades have smaller cross-section further radially outward gives the best efficiency in terms of moving a certain volume of air with the least horsepower, but limits performance in terms of moving enough air. Spinning faster to gain more air movement results in high tip speeds, but high tip speeds create structural problems and create noise that may exceed environmental guidelines. A further requirement for the fan blades is the need for a narrow swath. In a variable pitch fan, increase of the pitch of the fan blades results in the fan blades cutting a wider swath through the air. Hence, it is preferred to have narrow blades. On the other hand, it is known to be desirable to have a high solidity ratio, defined as the width of the outside perimeter of the blades divided by the total outside perimeter, since this provides higher performance in terms of total air flow.
Other problems faced in the design of variable pitch fans include design of easily detachable blades, reducing the size of the pitch shifting mechanism while maintaining stability of the pitch shifting mechanism and mounting counterweights on the fan blades.